BEIRUT, July 17 (Reuters) - Turkish-backed rebels clashed with Kurdish fighters from a U.S.-backed alliance in northwestern Syria on Monday in escalating violence between the two sides, officials and a monitoring group said.

The clashes took place around the village of Ain Daqna and the nearby Menagh air base north of Aleppo, while Turkish forces stepped up shelling positions in other areas, the officials and the Syrian Observatory for Human Rights said.

"There are big clashes happening on the Ain Daqna axis, between us and the Turks and their mercenaries," Rojhat Roj, a spokesman for the Kurdish YPG militia said.

Mostafa Bali of the Syrian Democratic Forces (SDF), a Kurdish and Arab force which is dominated by the YPG, said some Turkish-backed rebels had been killed and captured.

Both the YPG and SDF have received U.S. support, and are backed by Washington in its fight against Islamic State in Raqqa, further east.

The British-based Observatory, which monitors the war using sources on the ground, said the clashes and shelling were an attempt by the Turkish side to advance.

Syrian rebels said this month they were preparing to join the Turkish military in a major new offensive against Kurdish forces in northwestern Syria.

Turkey views the YPG as an extension of the Kurdish PKK group which is waging an insurgency in southeastern Turkey. Ankara's operation "Euphrates Shield", launched last year as an incursion into Syria to support rebel groups there, has focused on driving both Islamic State and the YPG away from the border.

The YPG controls vast stretches of land along the border northeast of Aleppo, and a pocket of territory to its northwest. In between, Turkish-backed factions have taken over territory to stop Kurdish forces linking up.

Clashes between the YPG and Turkey, both U.S. allies, have caused tension between Ankara and Washington. Turkey has criticised the U.S. decision to arm the YPG in its fight against Islamic State in Syria. (Reporting by John Davison; Editing by Richard Balmforth)